This invention relates to temperature control systems and, more particularly, to a novel and highly-effective water heater system wherein a combined set point and feed forward control is established that minimizes fluctuations in the temperature of the hot water by anticipating changes in BTU requirements.
Many hot water temperature control systems are known in the prior art. Often a thermostat is employed to establish a temperature set point and to control the heating of the water in such a manner as to maintain the temperature of the water as delivered by the system at this set point. Such a conventional thermostat system does not operate entirely satisfactorily in a water heater, however, in view of the layering of hot water over cold water in the storage tank. As hot water is withdrawn from the tank at the top and replaced by cold water at the bottom, the temperature sensor initially detects the desired high temperature and then suddenly, as the cold water reaches the level of the sensor, detects a temperature that may be 100.degree. F. or more below the set point. The thermostat then calls for more heat to be supplied to the water, but by that time much of the storage tank is filled with cold water.
This problem is not solved by placing the temperature sensor at the bottom of the storage tank, near the incoming water; such placement does not permit precise control of the temperature of the water at the top of the tank, as delivered to the appliances or other end users of the hot water. Moreover, regardless of the location of the sensor in prior art systems of this type, there is an abrupt change in the temperature of the water at the sensor, often of 100.degree. F. or more, as water is withdrawn from the storage tank. Since the system has no way of anticipating this abrupt change, it fails to call for heat as early as it should, and, when a call for heat is made, the system must run at maximum capacity for an extended period in order to catch up with demand.
The problem can be mitigated by providing a pump and associated plumbing for the purpose of mixing the heated and unheated water and deriving a signal based on the temperature of the mixture for controlling the BTU input to the storage tank. However, this proposal, put into practice in some commercial water heaters, has the serious drawbacks that it is costly and not entirely reliable. Specifically, the pump adds a significant increment to the price of the heating system and is subject to breakdown, leaving the system without a feedforward control.